The invention relates to a lubricant composition and methods for coating a substrate such as magnetic media with a lubricant.
Magnetic media is commonly used in the computer industry for storing large amounts of data. Magnetic recording occurs by moving magnetic media past a magnetic record head consisting of a small electromagnet with a gap. To record information on the magnetic media, a current is applied to the windings of the electromagnet creating a magnetic field in the gap region. The magnetic field affects the polarity of the magnetic materials in the magnetic media that are in close proximity to the head gap. Changing the direction of current flow can reverse the direction of magnetization and the polarity of the magnetic materials. To read information from magnetic media, a read head constructed similarly to the record head is brought into close proximity with the magnetic media. The magnetic field of the magnetic media induces a voltage in the read head. The voltage changes when the direction of the magnetic field from the magnetic media changes.
During normal operation, the magnetic media is moved or rotated relative to the record head with a small space between the media and the head. At the end of the recording process, the magnetic media is often in direct physical contact with the head. The frictional force produced can wear both the head and the magnetic media. Eventually, the frictional force can become large enough to damage either the media or the head.
To minimize the wear of the magnetic disk and head, a lubricant is placed on the surface of the magnetic media. The presence of the lubricant improves the durability of the magnetic media. Typically, the lubricant is a perfluoropolyether (PFPE) with functionalized end groups. Perfluoropolyether lubricants are chemically inert, thermally stable, moisture repellent compositions that possess relatively low surface tension, good lubricity and low volatility. As a result, they can be effective and long-lasting lubricants for magnetic media.
The trend in the computer industry is to increase the recording density. Increasing recording density can be achieved by increasing the output signal of the magnetic media. However, a lubricant layer between the record head and the magnetic material of the magnetic media diminishes the intensity of the signal that can be recorded or read. The decreased signal intensity is due, at least in part, to an increased distance between the head and the magnetic material due to the presence of the lubricant layer. Consequently, to maximize the output signal, a thin lubricant coating is often preferred. State-of-the-art magnetic media typically has a lubricant layer thickness below about 2 nm. The lubricant usually is applied as a dilute solution in a suitable solvent. After application of the lubricant composition, the solvent is evaporated leaving a thin, uniform lubricant coating.
Perfluoropolyethers have been extensively used as lubricants for magnetic media. Various perfluoropolyether lubricants have been described, for example, in U.S. Pat. No. 4,721,795 (Caporiccio et al.) and U.S. Pat. No. 5,049,410 (Johary et al.) Many perfluoropolyether lubricants contain a mixture of perfluoropolyether compounds with a variety of molecular weights and structures. These lubricants have limited solubility in most solvents.
A particularly effective solvent for perfluoropolyethers is 1,1,2-trichloro-1,2,2-trifluoroethane. This chlorofluorocarbon solvent offers the additional advantage of being relatively volatile so it can be removed readily after application of the lubricant composition to magnetic media. However, the 1987 Montreal Protocol calls for reductions in the use of chlorofluorocarbons to minimize degradation of the stratospheric ozone layer.
Certain perfluorinated alkanes have been used in place of chlorofluorocarbons as perfluoropolyether solvents such as those described in U.S. Pat. No. 4,721,795 (Caporiccio et al.). Additionally, U.S. Pat. No. 5,049,410 (Flynn et al.) discloses the use of a perfluorinated, nonaromatic cyclic organic solvent for dissolution of polyfluoropolyether lubricants. However, some of these compounds tend to have relatively long atmospheric lifetimes and can potentially contribute to global warming.
Thus, there is a need for solvents with short atmospheric lifetimes that dissolve polyfluoropolyether lubricants. The invention provides fluorinated ketone solvents with these desirable characteristics.
The invention provides a lubricant composition comprising about 10 to about 10,000 ppm perfluoropolyether lubricant and about 90 to about 99.9 weight percent fluorinated ketone solvent based on the weight of the lubricant composition. The lubricant compositions can further comprise about 0.1 to about 1000 ppm of an additive such as a cyclic phosphazene compound. The lubricant composition typically has low solubility for possible contaminants such as water, silicones, and general hydrocarbons. Additionally, the lubricant composition can have low global warming potential.
The fluorinated ketone solvent of the invention typically has a total of 5 to 10 carbon atoms. In some embodiments, the fluorinated ketone has 6 to 8 carbon atoms. The solvent can be a perfluoroketone, a compound in which all of the hydrogen atoms on the carbon backbone are replaced with fluorine. Alternatively, the fluorinated ketone solvent can have up to two hydrogen atoms and up to two non-fluorine halogen atoms including bromine, chlorine, and iodine attached to the carbon backbone. One or more heteroatoms can interrupt the carbon backbone of the molecule.
More than one fluorinated ketone solvent can be used in the lubricant composition. In some embodiments, one or more miscible solvents can replace a portion of the fluorinated ketone solvent. For example, up to about 10 weight percent of the fluorinated ketone can be replaced with another miscible solvent.
Typically, the perfluoropolyether lubricant comprises a perfluoropolyether molecule represented by the formula:
A"Brketopenst"(CyF2y)O(C4F8O)k(C3F6O)m(C2F4O)n(CF2O)p(CzF2z)"Brketclosest"Axe2x80x2xe2x80x83xe2x80x83I
where y and z are integers independently ranging from 0 to about 20. The variables k, m, n, and p are integers independently ranging from 0 to about 200; the sum of k, m, n, and p ranges from 2 to about 200. Groups A and Axe2x80x2 are independently selected monovalent organic moieties.
Another aspect of the invention provides a method of lubricating a substrate. The method comprises applying a coating of a lubricant composition to a substrate followed by drying the coating to form a lubricant film on the surface of the substrate. Typically, the substrate is magnetic media such as a thin film or hard disk.
The invention relates to a lubricant composition and methods of coating a substrate with a lubricant. In particular, the invention provides a lubricant composition comprising about 10 to about 10,000 ppm of a perfluoropolyether lubricant and about 90 to about 99.9 weight percent fluorinated ketone solvent based on the weight of the lubricant composition. The lubricant composition can be applied to form a thin, uniform layer of lubricant on a substrate. The substrate is typically magnetic media such as a thin film or hard disk.
The fluorinated ketone solvent of the invention typically has a total of 5 to 10 carbon atoms. In some embodiments, the fluorinated ketone has 6 to 8 carbon atoms. The fluorinated ketone solvent typically has a boiling point less than about 150xc2x0 C. In some embodiments, the boiling point is less than about 100xc2x0 C. To provide adequate solvency for the perfluoropolyether lubricant, the fluorinated ketone solvent is highly fluorinated. The solvent can be a perfluoroketone, a compound in which all of the hydrogen atoms on the carbon backbone are replaced with fluorine. Alternatively, the fluorinated ketone solvent can have up to two hydrogen atoms and up to two non-fluorine halogen atoms including bromine, chlorine, and iodine attached to the carbon backbone.
Representative examples of perfluorinated ketone compounds suitable as solvents for perfluoropolyether lubricants include CF3(CF2)5C(O)CF3, CF3C(O)CF(CF3)2, CF3CF2CF2C(O)CF2CF2CF3, CF3CF2C(O)CF(CF3)2, (CF3)2CFC(O)CF(CF3)2, (CF3)2CFCF2C(O)CF(CF3)2, (CF3)2CF(CF2)2C(O)CF(CF3)2, (CF3)2CF(CF2)3C(O)CF(CF3)2, CF3(CF2)2C(O)CF(CF3)2, CF3(CF2)3C(O)CF(CF3)2, CF3(CF2)4C(O)CF(CF3)2, CF3(CF2)5C(O)CF(CF3)2, CF3CF2C(O)CF2CF2CF3, perfluorocyclopentanone, and perfluorocyclohexanone.
Representative examples of fluorinated ketones with either one or two atoms other than fluorine attached to the carbon backbone include CHF2CF2C(O)CF(CF3)2, CF3C(O)CH2C(O)CF3, (CF3)2CFC(O)CF2Cl, CF2ClCF2C(O)CF(CF3)2, CF2Cl(CF2)2C(O)CF(CF3)2, CF2Cl(CF2)3C(O)CF(CF3)2, CF2Cl(CF2)4C(O)CF(CF3)2, CF2Cl(CF2)5C(O)CF(CF3)2, and CF2ClCF2C(O)CF2CF2CF3.
The fluorinated ketone can also contain one or more heteroatoms interrupting the carbon backbone. Suitable heteroatoms include, for example, nitrogen, oxygen and sulfur atoms. Representative compounds include CF3OCF2CF2C(O)CF(CF3)2, CF3OCF2C(O)CF(CF3)2, and the like.
Fluorinated ketones can be prepared by known methods. One approach involves the dissociation of perfluorinated carboxylic acid esters of the formula RfCO2CF(Rfxe2x80x2)2 with a nucleophilic initiating agent as described in U.S. Pat. No. 5,466,877 (Moore). Rf and Rfxe2x80x2 are fluorine or a perfluoroalkyl group. The fluorinated carboxylic acid ester precursor can be derived from the corresponding fluorine-free or partially fluorinated hydrocarbon ester by direct fluorination with fluorine gas as described in U.S. Pat. No. 5,399,718 (Costello et al.).
Perfluorinated ketones that are alpha-branched to the carbonyl group can be prepared as described in U.S. Pat. No. 3,185,734 (Fawcett et al.). Hexafluoropropylene is added to acyl halides in an anhydrous environment in the presence of fluoride ion. Small amounts of hexafluoropropylene dimer or trimer impurities can be removed by distillation from the perfluoroketone. If the boiling points are too close for fractional distillation, the dimer or trimer impurity can be removed by oxidation with alkali metal permanganate in a suitable organic solvent such as acetone, acetic acid, or a mixture thereof. The oxidation reaction is typically carried out in a sealed reactor at ambient or elevated temperatures.
Linear perfluorinated ketones can be prepared by reacting a perfluorocarboxylic acid alkali metal salt with a perfluorocarbonyl acid fluoride as described in U.S. Pat. No. 4,136,121 (Martini et al.) Such ketones can also be prepared by reacting a perfluorocarboxylic acid salt with a perfluorinated acid anhydride in an aprotic solvent at elevated temperatures as described in U.S. Pat. No. 5,998,671 (Van Der Puy).
All the above-mentioned patents describing the preparation of fluoroketones are incorporated by reference in their entirety.
More than one fluorinated ketone solvent can be used in the lubricant composition. In some embodiments, one or more miscible solvents can replace a portion of the fluorinated ketone solvent. For example, up to about 10 weight percent of the fluorinated ketone can be replaced with another miscible solvent. Suitable miscible solvents include, for example, hydrofluorocarbons, hydrochlorofluorocarbons, perfluorocarbons, perfluoropolyethers, hydrofluoroethers, hydrochlorofluoroethers, hydrofluoropolyethers, fluorinated aromatic compounds, chlorofluorocarbons, bromofluorocarbons, bromochlorofluorocarbons, hydrobromocarbons, iodofluorocarbons, and hydrobromofluorocarbons. In some embodiments, the co-solvent includes hydrofluorocarbons, hydrofluoroethers, hydrochlorofluorocarbons, perfluorocarbons, chlorofluorocarbons, bromofluorocarbons, bromochlorofluorocarbons, hydrobromofluorocarbons, and mixtures thereof. In other embodiments, hydrofluorocarbons, hydrofluoroethers, hydrochlorofluorocarbons, perfluorocarbons, and hydrobromofluorocarbons are utilized. These co-solvents typically contain from 5 to 10 carbon atoms and have a boiling point less than about 150xc2x0 C. or less than about 100xc2x0 C. Representative solvents include CF3CH2CF2CH3, C5F11H, C6F13H, CF3CFHCFHCF2CF3, C6F14, C7F16, C8F18, (C4F9)3N, perfluoro-N-methylmorpholine, C4F9OCH3, F(C3F6O)2CF2H, HCF2O(CF2O)m(CF2CF2O)nCF2H (where m is an integer from 0 to 8; n is an integer from 0 to 4; and the sum of n plus m is from 0 to 8), F(C3F6O)2CF2H, C3F7CF(OC2H5) CF(CF3)2, C4F9OC2H5, C4F9-c-C4F7O, and C3F5HCl2.
The fluorinated ketones typically have low solubility for impurities usually associated with a lubrication deposition system. These impurities include, for example, dioctylphthalates, silicones, water, and general hydrocarbons. If these impurities are coated on the magnetic media, they can adversely affect the performance of the magnetic media. A solvent with low solubility for impurities can result in lubricant coatings with fewer contaminates. Additionally, the useful lifetime of the lubrication deposition system can be extended through the use of a solvent with low solubility for impurities. When the concentration of the impurities in the lubrication deposition system reaches an unacceptable level, the contaminated solvent is generally discarded and replaced with fresh solvent. Extending the lifetime of the lubrication deposition system can potentially decrease cost associated with the lubrication deposition process and the amount of waste generated.
Perfluoroketones, though fully fluorinated, have a much lower global warming potential (GWP) than perfluorocarbons, i.e. fully fluorinated hydrocarbons that do not contain ketone groups. As used herein, xe2x80x9cGWPxe2x80x9d is a relative measure of the warming potential of a compound based on the structure of the compound. The GWP of a compound, as defined by the Intergovernmental Panel on Climate Change (IPCC) in 1990 and updated in 1998 (World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Projectxe2x80x94Report No. 44, Geneva, 1999), is calculated as the warming due to the release of 1 kilogram of a compound relative to the warming due to the release of 1 kilogram of CO2 over a specified integration time horizon (ITH):             GWP      x        ⁢          (              t        xe2x80x2            )        =                    ∫        0        ITH            ⁢                        F          x                ⁢                  C          ox                ⁢                  e                                                    -                t                            /              τ                        ⁢                          xe2x80x83                        ⁢            x                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                                    ∫        0        ITH            ⁢                        F                      CO            2                          ⁢                              C                          CO              2                                ⁢                      (            t            )                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                    
where F is the radiative forcing per unit mass of a compound (the change in the flux of radiation through the atmosphere due to the IR absorbance of that compound), C is the atmospheric concentration of a compound, xcfx84 is the atmospheric lifetime of a compound, t is time and x is the compound of interest (i.e., C0x is the time 0 or initial concentration of compound x).
The commonly accepted ITH is 100 years representing a compromise between short term effects (20 years) and longer term effects (500 years or longer). The concentration of an organic compound in the atmosphere is assumed to follow pseudo first order kinetics (i.e., exponential decay). The concentration of CO2 over that same time interval incorporates a more complex model for the exchange and removal of CO2 from the atmosphere (the Bern carbon cycle model).
CF3CF2C(O)CF(CF3)2 has an atmospheric lifetime of approximately 5 days based on photolysis studies at 300 nm. Other perfluoroketones show similar absorbances and thus are expected to have similar atmospheric lifetimes. A measured IR cross-section was used to calculate the radiative forcing value for CF3CF2C(O)CF(CF3)2 using the method of Pinnock, et al. (J. Geophys. Res., 100, 23227, 1995). Using this radiative forcing value and the 5 day atmospheric lifetime, the GWP (100 year ITH) for a perfluoroketone with 6 carbon atoms is 1 while the GWP for C2F6 is 11,400. The fluorinated ketones of the invention typically have a GWP less than about 10. As a result of their rapid degradation in the lower atmosphere, the fluorinated ketones have short lifetimes and would not be expected to contribute significantly to global warming.
Additionally, the fluorinated ketones have low toxicity. For example, the perfluoroketone CF3CF2C(O)CF(CF3)2 has low acute toxicity based on short-term inhalation tests with rats. The xe2x80x9cno-effectxe2x80x9d level for cardiac sensitization for CF3CF2C(O)CF(CF3)2 is  greater than 150,000 ppm, comparable to the xe2x80x9cno-effectxe2x80x9d level for perfluorohexane (C6F14), a perfluorocarbon with a comparable number of carbon atoms that has a long history of safe use.
The perfluoropolyether lubricant includes one or more perfluoropolyether compounds containing the repeating unit "Parenopenst"CaF2aO"Parenclosest" in which a is an integer from 1 to about 8 or from 1 to about 4. These repeating units can be linear or branched. Many of the perfluoropolyether lubricants useful in the invention have been described previously such as in U.S. Pat. No. 4,671,999 (Burguette et al.), U.S. Pat. No. 4,268,556 (Pedrotty), U.S. Pat. No. 4,803,125 (Takeuchi et al.), U.S. Pat. No. 4,721,795 (Caporiccio et al.), U.S. Pat. No. 4,746,575 (Scaretti et al.), U.S. Pat. No. 4,094,911 (Mitsch et al.), and U.S. Pat. No. 5,663,127 (Flynn et al.). These patents are hereby incorporated by reference.
Typically, the perfluoropolyether lubricant is a liquid at room temperature. Such a lubricating liquid, otherwise known as a fluid, can have a wide range of viscosities. In some embodiments, the lubricant is a viscous oil. The molecular weight is usually high enough to prevent volatilization or removal of the lubricant from the substrate during use. When the substrate is a magnetic disk, the molecular weight of the lubricant is usually high enough to prevent the removal of the lubricant by centrifugal forces created when the disk is rotated relative to either the read or record head.
Typically, the perfluoropolyether lubricant can be represented by the formula:
A"Brketopenst"(CyF2y)O(C4F8O)k(C3F6O)m(C2F4O)n(CF2O)p(CzF2z)"Brketclosest"Axe2x80x2xe2x80x83xe2x80x83I
where y and z are independent integers from 0 to about 20; the variables k, m, n, and p are independent integers from 0 to about 200; and the sum of k, m, n, and p is from 2 to about 200. The repeating units can be randomly distributed in the backbone of the lubricant molecule. Each of the groups CyF2y, CzF2z, C4F8O, C3F6O, and C2F4O in Formula I can be linear or branched. The A and Axe2x80x2 end groups are independently selected monovalent organic moieties that have from 1 to 20 carbon atoms. The end groups can be either hydrogen-containing or nonhydrogen-containing and can include heteroatoms such as oxygen, nitrogen, sulfur, or a halogen other than fluorine.
In some embodiments, a major amount of the lubricant includes perfluoropolyether compounds containing at least one hydrogen-containing end group. In this embodiment, minor amounts of the lubricant can include compounds having only nonhydrogen-containing end groups. In another embodiment, a major amount of the lubricant includes perfluoropolyether compounds containing two hydrogen-containing end groups. In this embodiment, minor amounts of the lubricant can include compounds having only one hydrogen-containing end group or two nonhydrogen-containing end groups.
Nonhydrogen-containing A and Axe2x80x2 groups include, for example, xe2x80x94CF2CF3, xe2x80x94CF3, xe2x80x94F, xe2x80x94OCF2CF3, xe2x80x94CF3, xe2x80x94CF2C(O)F, and xe2x80x94C(O)F. An example of a perfluoropolyether with nonhydrogen-containing end groups is: 
where m is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000. This type of lubricant is commercially available as KRYTOX(trademark) 142 from E. I. Dupont deNemours and Company of Wilmington, Del.. Other nonhydrogen-containing perfluoropolyether lubricants include, for example, certain types of FOMBLIN(trademark) fluids such as FOMBLIN(trademark) Y and Z (available from Montedison S.p.A. of Milan, Italy) as well as certain types of DEMNUM(trademark) fluids such as DEMNUM(trademark) SA and SP (available from Dalkin Industries, Ltd. of Tokyo, Japan).
Examples of hydrogen-containing A and Axe2x80x2 groups are alkyl, aryl, and alkaryl groups, which can be partially substituted with fluorine atoms and can contain heteroatoms, such as oxygen, sulfur, and nitrogen, for example. Particularly useful examples of such hydrogen-containing end groups include:
(a) xe2x80x94Bxe2x80x94D groups wherein:
(i) B is: xe2x80x94CH2Oxe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94, xe2x80x94CF2xe2x80x94, and xe2x80x94CF2Oxe2x80x94; and
(ii) D is: 
wherein R and R1 are independently alkyl groups having 1 to 3 carbon atoms; G is a divalent alkyl group having 1 to 5 carbon atoms; and E is xe2x80x94H, xe2x80x94OH, xe2x80x94OCH3, xe2x80x94OC2H5, or xe2x80x94OC3H7 (each R, R1, and G group can be substituted with one or more halogen atoms);
(b) xe2x80x94(CtH2t)SH, xe2x80x94(CtH2t)SR2, xe2x80x94(CtH2t)NR22, xe2x80x94CO2R2, xe2x80x94(CtH2t)CO2H, xe2x80x94(CtH2t)SiR2zQ3z, xe2x80x94(CtH2t)CN, xe2x80x94(CtH2tt)NCO, xe2x80x94(CtH2t)CHxe2x95x90CH2, 
xe2x80x83xe2x80x94(CtH2txe2x80x94)CO2R2, xe2x80x94(CtH2t)OSO2CF3, xe2x80x94(CtH2t)OC(O)Cl, xe2x80x94(CtH2t)OCN, xe2x80x94(O)COC(O)xe2x80x94R2, xe2x80x94(CtH2t)X, xe2x80x94CHO, xe2x80x94(CtH2t)CHO, xe2x80x94CH(OCH3)2, xe2x80x94(CtH2t)CH(OCH3)2, xe2x80x94(CtH2t)SO2Cl, xe2x80x94C(OCH3)xe2x95x90NH, xe2x80x94C(NH2)xe2x95x90NH, xe2x80x94(CtH2t)OC(O)CHxe2x95x90CH2, xe2x80x94(CtH2t)OC(O)C(CH3)xe2x95x90CH2, 
xe2x80x83xe2x80x94(CtH2t)OR2, xe2x80x94(CtH2t)OC(O)R2, xe2x80x94(CtH2t)(CtH2tO)xH, 
wherein Q is xe2x80x94OH, xe2x80x94OR3, xe2x80x94H, xe2x80x94Cl, xe2x80x94F, xe2x80x94Br, or xe2x80x94I; R2 is hydrogen, an aryl group containing 6 to 10 carbons, or an alkyl group containing 1 to 4 carbons; R3 is an alkyl group containing 1 to 4 carbons; X is Cl, Br, F, or I; z is an integer ranging from 0 to 2; x is an integer ranging from 1 to 10; v is an integer ranging from 0 to 1; and t is an integer ranging from 1 to 4;
(c) xe2x80x94OCR4R5R6, where in R4 is hydrogen, an alkyl or fluoroalkyl group containing 1 to 4 carbons; R5 is hydrogen or an alkyl group containing 1 to 4 carbons; and R6 is fluoroalkyl group containing 1 to 4 carbon atoms; and 
where t is defined as above.
Specific examples of particularly preferred perfluoropolyethers having functional end groups according to formula I include: 
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as FOMBLIN(trademark) Z-DEAL from Montedison S.p.A. of Milan, Italy); 
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (examples of such compounds are commercially available as FOMBLIN(trademark) AM 2001 and AM 3001 from Montedison S.p.A. of Milan, Italy);
(c) HOCH2xe2x80x94CF2xe2x80x94Oxe2x80x94(CF2CF2O)nxe2x80x94(CF2O)pxe2x80x94CF2xe2x80x94CH2OH
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as FOMBLIN(trademark) Z-DOL from Montedison S.p.A. of Milan, Italy); 
wherein k is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000;
(e) HOCH2xe2x80x94C3F6xe2x80x94Oxe2x80x94(C4F8O)kxe2x80x94C3F6xe2x80x94CH2OH
wherein k is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000; 
wherein n is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000;
(g) HOCH2CF2O"Parenopenst"CF2CF2O"Parenclosest"nCF3 
wherein n is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000; 
wherein n is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000; 
wherein m is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as DEMNUM(trademark) ester from Daikin Industries, Ltd.);
(j) CF3CF2CF2O"Parenopenst"CF2CF2CF2O"Parenclosest"mCF2CF2CH2OH
wherein m is an integer having a value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as DEMNUM(trademark) alcohol from Daikin Industries, Ltd.);
(k) OCNCH2xe2x80x94CF2xe2x80x94Oxe2x80x94(CF2CF2O)nxe2x80x94CF2O)pxe2x80x94CF2xe2x80x94CH2NCO
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as FOMBLIN(trademark) Z-DISOC from Montedison S.p.A. (Milan, Italy));
(l) H(OH4C2)1.5OCH2xe2x80x94CF2xe2x80x94Oxe2x80x94(CF2CF2O)nxe2x80x94(CF2O)pxe2x80x94CF2xe2x80x94CH2O(C2H4O)1.5H
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as FOMBLIN(trademark) Z-DOL-TX from Montedison S.p.A. (Milan, Italy)); 
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as FOMBLIN(trademark) Z-TETRAOL from Montedison S.p.A. (Milan, Italy)): and 
wherein n and p are integers, each having an independent value such that the lubricant has a number average molecular weight in the range of 1000 to 5000 (an example of such a compound is commercially available as FOMBLIN(trademark) Z-DIAC from Montedison S.p.A. of Milan, Italy).
Methods of making compounds according to the formulae listed as examples (d) to (h) are described in U.S. Pat. No. 5,039,432 (Ritter et al.), which is incorporated by reference.
In some embodiments, the lubricant compositions further comprises various additives. Suitable additives include, for example, cyclic phosphazene compounds such as Dow X-1P and X-100 (available from Dow Chemical of Midland, Mich.). The additives can enhance the performance of the lubricant, for example, by reducing the rate of lubricant breakdown and wear. The additives are usually added at levels from about 0.1 ppm to about 1000 ppm based on the weight of the lubricant composition. In other embodiments, the additives are present in concentrations from about 1 ppm to about 300 ppm or from about 10 ppm to about 250 ppm.
Another aspect of the invention provides a method of lubricating a substrate. The method comprises applying a coating of a lubricant composition to a substrate followed by removing the solvent from the coating to form a neat lubricant film. The lubricant composition comprises about 10 to about 10,000 ppm perfluoropolyether lubricant and about 90.0 to about 99.9 weight percent fluorinated ketone solvent based on the weight of the lubricant composition. The fluorinated ketone solvent is removed during the drying step. Typically, the substrate is magnetic media including, for example, thin films and hard disks. The magnetic media typically consists of a base layer such as glass, aluminum or a polymeric material and a magnetic layer containing iron, cobalt, nickel, or the like. The magnetic media can contain optional layers of carbon or other materials to enhance, for example, durability and performance of the media. The lubricant is usually applied as the outermost layer.
To meet the demands for increased data storage densities, the magnetic recording industry has had to develop magnetic media with significantly higher signal output levels. The higher signal output levels have been achieved, at least in part, by providing smoother, lower defect magnetic media containing thinner lubricant coatings. The decreased thickness of the lubricant coating allows the magnetic head to be in closer proximity to the magnetic material in the media. However, if the thickness of lubricant coating is too thin, the durability of the magnetic media can be compromised. The thickness of the lubricant layer is typically less than about 2 nm in state-of-the-art magnetic media.
Although the lubricant composition can be applied to the substrate by any known process, two methods are widely used for application of lubricants to hard disks. The first method involves placing a hard disk in a coating chamber. The lubricant composition is pumped into the coating chamber to completely cover the disk. The lubricating composition is then drained from the chamber at a controlled rate leaving a uniform coating on surface of the disk. The second application method involves dipping a hard disk into a vessel containing the lubricant composition and then slowly pulling the disk back out.
With either the draining or dipping application method, the thickness of the lubricant coating can be controlled by varying the concentration of the lubricant in the lubricant composition and the speed of either draining the lubricant composition or pulling the disk out of the lubricant composition. Lowering the concentration of the perfluoropolyether in the lubricant composition can decrease the thickness of the lubricant coating. Similarly, either decreasing the rate of removal of the hard disk from the lubricant composition using the draining technique or decreasing the rate of removal of the hard disk from the lubricant composition using the dipping technique can decrease the thickness of the lubricant coating.
The fluorinated ketone solvent can be removed, for example, by drying or evaporating at ambient or higher temperatures. Temperatures up to about 150xc2x0 C. can be used for solvent removal. The rate of removal can be increased through the use of a non-reactive gas such as, for example, nitrogen or argon to assist evaporation of the solvent. As the solvent is removed, the lubricant forms a uniform film over the substrate.